1. Field of the Invention
The present invention relates to a toner which is useful for the development of electrostatic images. More particularly, the invention relates to a metal-free toner having excellent charging characteristics.
2. Discussion of Background
Developers for use in electronic duplicators must be capable of sticking to a photoreceptor or to any image carrier having an electrostatic image formed thereon. In order to provide a hard copy of the image, the developer is transferred from the photoreceptor or image carrier to an image receiving sheet where it is fixed. Prior art developers generally fall into one of two categories: two-component developers comprising a carrier and a toner and one-component developers (magnetic toners) not containing any carrier.
An important characteristic of a toner is its charging property. That is, to be useful, a toner must be capable of being charged, positively or negatively, to a suitable level by virtue of its contact with a carrier or with a developer tank wall. Further, the toner charge level must remain stable during continuous use or under extreme conditions.
The charging property of a toner may be affected by the presence of a binder resin and a colorant, but the presence of these materials is often insufficient to provide the desired level of performance. The incorporation of certain agents in toners (i.e., charge controlling agents) for modifying charging properties is known, and the agents include, for example, positive charging nigrosine dyes and quaternary ammonium salts and negative charging metal-containing monoazo dyes, salicylic acid-metal complexes and copper phthalocyanine pigments.
Unfortunately, these conventional charge controlling agents have several problems associated with them that limit their effectiveness in providing toners with adequate charging properties and other important toner characteristics.
One problem encountered with prior art charge controlling agents is that they increase the safety risk associated with toners containing them. Almost all conventional charge controlling agents, especially negative charge controlling agents, are metal-containing dyes containing, for example, chromium metal. Since a toner is used in places which are often in extremely close proximity to humans, the presence of toxic metals in toners is dangerous to office workers, etc.. The recent trend towards safe workplaces, etc., and the desire to eliminate dangerous and toxic substances from areas with high human traffic has created a need for a charge controlling agent for toners which does not contain toxic metals like chromium but which has charging properties as good as or better than conventional agents.
A further problem encountered with prior art charge controlling agents is that the charging stability provided by the agents is poor. While many of the conventional charge controlling agents provide a high initial charging level, they do not provide sufficient charging stability over time. Therefore, when these agents are used under continuous duplication or continuous printing conditions, the charging level of the toners incorporating these agents varies with time, providing uneven copies and stained copies. With the increase in demand for rapid-processing duplicators capable of continuously and rapidly duplicating large amounts of copies over long periods of time this problem is growing larger and larger. It was with these considerations in mind that the inventors undertook the development of a charge controlling agent having reduced toxicity and improved charging stability.